In the event that a vehicle has hydraulically motivated power steering and that the engine on the vehicle stops or the pump supplying the hydraulic pressure fails for any reason, the vehicle will continue moving due to its momentum or from its normal operating transmission. Because the steering is hydraulically actuated, it is essential that steering control be retained. In systems wherein the steering is provided via an open centered steering valve, accumulators have been utilized which are charged by the pump when it is operating and which discharge fluid to the steering valve when the pump fails. Examples of such systems are found in U.S. Pat. No. 3,882,952 issued May 13, l975 to E. R. Crabb, and U.S. Pat. No. 3,458,998 issued Aug. 5, l969 to A. E. Bishop.
Such systems as have been used with open centered control valves have not, however, been readily adaptable for use with closed center control valves. Thus, with closed center control valves, auxiliary wheel driven pumps such as those taught in U.S. Pat. No. 3,613,818 issued Oct. 19, 1971 to G. E. Schubert, et al. and in U.S. Pat. No. 3,952,510 issued Apr. 27, l976 to W. A. Peterson, have been utilized to supply pressure to the control valve when the engine and/or main pump have failed. It is clear that as the vehicle rolls the wheels are turned and, hence, the auxiliary pumps are turned to supply the required emergency pressure. This is, however, a relatively expensive solution to the problem of retaining control in an emergency situation, and, since it utilizes an extra pump, it introduces the possible problem of failure of this extra pump. Thus, a fully adequate and inexpensive solution to the problem of providing emergency steering does not necessarily result from utilizing a wheel-driven pump.
Accumulators have been utilized with closed centered steering valves to supply power thereto when the main pump fails. For example, U.S. Pat. No. 3,570,519 issued Mar. 16, 1971 to D. L. Bianchetta and U.S. Pat. No. 3,918,847 issued Nov. 11, 1975 to J. A. Junck, et al., both disclose such systems. The systems as disclosed in the two just mentioned patents, however, have both the pump and accumulator operating on the same line and through a single reducing valve to supply pressure to the closed centered steering valve. Since the accumulator and pump are in the same line, the accumulator pressure is always as high as that in the pump. Thus, when the pump fails the operator of the vehicle does not feel an immediate sharp difference in steering, but instead only feels a gradual deterioration thereof. Accordingly, should warning lights or the like fail to properly indicate when pump operation has been retarded or stopped, the vehicle operator may not receive sufficient immediate warning to allow him to steer the vehicle to a safe stopping place.